Their major evolutionary advantage was the progressive development of a chewing apparatus that became the most sophisticated ever developed by a reptile. They had beaks, multiple teeth rows, cheek pouches and true chewing.[1] Efficient chewing means faster digestion, as the material is broken into smaller pieces.

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Ornithopoda means "bird feet", from the Greek. It refers to their three-toed feet (though many early forms had four toes). They had no armour, a horny beak, and various other features. Some ornithopods and cerapods had thin cartilaginous plates along the outside of the ribs. In some cases, these plates mineralized and so were fossilized. The function of these plates is unknown.[2][3]

The ornithopods became one of the most successful groups of herbivores in the Cretaceous world, and dominated the landscape. They started out as small, bipedal running browsers, and grew in size and numbers. Hypsilophodon from the Lower Cretaceous of the Isle of Wight, England, is a typical example.[4] As they grew in size, they could move on four legs but still kept the habit of standing up on their hind legs when needed to reach food or perhaps to defend themselves or to run.

Their success is a perhaps surprising, as they had little obvious defence against theropodcarnivores. They could run, and had the usual defences of a herd animal.[1] They had horny beaks and some cartilagerib plates.[2][3]Iguanadonts, however, could protect themselves. They were very robust (heavy and strong), and armed with a dagger-like thumb.

Sizes of the largest ornithopods

The early ornithopods were only about 1 metre (3 feet) long, but probably very fast. They had a stiff tail, like the theropods, to help them balance as they ran on their hind legs. Later ornithopods became more adapted to grazing on all fours; their spines curved, and came to resemble the spines of modern ground-feeders like the bison. As they became more adapted to eating while bent over, they became semi-quadrupedal; still running on two legs, and comfortable reaching up into trees; but spending most of their time walking or grazing while on all fours.

Later ornithopods became larger, but never reached the huge size of the long-necked, long-tailed sauropods which they largely replaced. The largest, like Shantungosaurus, were as heavy as medium sized sauropods, but never grew much beyond 15 metres (50 feet).